Created attachment 651740[details][diff][review]
patch
Giving -moz-tree-cell-text a padding-bottom of 2px centers the text. On Win7 under default themes removing it again lets the text still centered on Aero and Basic.

Comment on attachment 651740[details][diff][review]
patch
I was a bit torn over padding-bottom: auto; in messenger-aero.css, but since it's supposed to override the one in messenger.css, I think it makes sense.